What Is The Contribution Of Abrasion To The Sediment Budget Of A Mixed Sand And Gravel Beach?
What is the contribution of abrasion to the sediment budget of a mixed sand and gravel beach? This was the primary research question for this thesis. Research focused on determining the abrasion rate (%/m2/d) and abrasion loss (kg/m2/d) and the factors that influence the abrasion of sediment using the Waitaki coast of the South Island of New Zealand. Laboratory tumbler experiments showed that abrasion of sediments along the Waitaki coast varied from place to place due to the intrinsic factors, of sediment size, textural mix, and degree of weathering. The influence of particle shape on abrasion was less clear. The results of sediment tracking with Radio Frequency Identification technology showed that the further the pebbles moved, the greater the resulting abrasion. However results from tagged pebbles did not adequately represent the continuous abrasion loss across the active beach. Consequently a new approach using abrasion baskets was developed to measure abrasion rates continuously in the swash zone. The result from abrasion baskets showed the greatest abrasion occurred in the middle of swash zone while less abrasion occurred at the upper swash limit and breaker zone. From these results using abrasion baskets, the concept of the ‘abrasion zone’ was defined in this study. A novel abrasion model was developed using swash forces on sediment as the driver of abrasion. This differs from traditional abrasion models where transport distance is the primary factor determining abrasion. The model conceptualizes abrasion as a continuous process in the abrasion zone, unlike previous research that has focused on abrasion rates of individual particles that experience episodic abrasion during transport and burial. Significantly, the model allows calculation of abrasion loss using abrasion zone width, abrasion rate (%/m2/d) and abrasion loss (kg/m2/d) occurring over 33 years from 1979 to 2011, for the Waitaki coast. A sediment budget model was developed for the Waitaki coast. Historical beach profiles were used to calculate rates of cliff retreat and longshore sediment transport. An empirical formula was used for simulating the bedload contribution of rivers. The abrasion loss was calculated from the abrasion model. The sediment budget showed that abrasion loss on the Waitaki coast varied from 474,000 to 700,000 m3/yr over the 33 years. Results showed that abrasion made the Waitaki coast erosional in 21 of the 33 years, and in the other 12 years, the sediment budget was positive due to cliff erosion supplying sediment to beaches. Longshore sediment transport and river supply were minor contributors at 3 % and 4 % of the budget respectively. Cliff erosion and abrasion loss dominated at 34 % and 59 % of the budget respectively during the 33 years. Thus abrasion is the most important (negative) contributor to the sediment budget of the mixed sand and gravel beaches studied.
Advisor: Stephenson, Wayne; Hilton, Michael
Degree Name: Doctor of Philosophy
Degree Discipline: Geography
Publisher: University of Otago
Keywords: Abrasion; Mixed sand and gravel beach; Sediment budget; Abrasion basket; Abrasion zone; Tumbler; RFID
Research Type: Thesis